Heart Physiology: Study with Video Lessons, Practice Problems & Examples

The heart's pumping mechanism involves the cardiac cycle, which consists of systole (contraction) and diastole (relaxation). Blood flows from veins into the atria, then into the ventricles, which pump it into arteries. Electrical signals, or action potentials, generated by the sinoatrial node, propagate through gap junctions and intercalated discs, coordinating contractions. Blood pressure is measured as systolic (during contraction) and diastolic (during relaxation), with optimal values being <120 mmHg and <80 mmHg, respectively. Hypertension, or high blood pressure, can lead to serious cardiovascular diseases like atherosclerosis and heart attacks.

Heart Physiology

Hi. In this video, we'll be looking at the physiology of the heart and examining how the heart pumps blood. Now, the heart circulates blood generally by filling up its chambers from the veins and then pushing that blood through the arteries. Now, the veins are going to empty into chambers known as atria. These are thinner, less muscular chambers than the ventricles. And the ventricles, which are more muscular than the atria, are going to be the powerful pumping stations that push that blood through the arteries of the body. So really the atria are there to receive blood and move it into the ventricles, and the ventricles are the guys who do the real heavy lifting and actually push that blood into the vasculature. Now the act of pumping by the heart is known as the cardiac cycle. This is a complete cycle of pumping out blood and filling up with blood. It's divided into 2 phases known as systole and diastole. Systole is the contraction phase where the muscles of the heart are going to contract and blood is ultimately going to be pumped. Diastole is the relaxation phase, which is going to allow the heart to actually fill with blood. Now, let me jump out of the image here and let's talk about what's going on with our atria and our ventricles during these two phases. Now when the atria and ventricles are in diastole, blood is going to flow into those atria and ventricles. So here we are in diastole, and you can see that blood, this is our superior vena cava. Here's our inferior vena cava. These are vena cavae. Right? So from the vena cavae, blood is going to enter into our, let me actually switch colors here so it's easier to see. On the blue background, this is our right atrium, and this is our right ventricle, so blood is going to flow into these. And on the other side, we have our left atrium, and left ventricle, and of course, the left atrium is going to be receiving blood from the pulmonary vein. So these chambers are going to fill up. Now, the atria are going to experience systole before the ventricles do. So the atria are going to be in systole, the ventricles in diastole, and what this is going to allow for is the blood in the atria to be pushed through those AV valves, those atrioventricular valves, and fill up the ventricles. Then the atria are going to go into diastole, and the ventricles are going to go into systole. And that's going to push the blood from the ventricles, and you can see this over here, push the blood from the ventricles into the arteries through those semilunar valves. So this whole process is recorded in electrical signals that look like this. Right? If you've ever seen a doctor show or a movie where there's a scene in a hospital there's always a machine in there that has like a little green screen or something with this little line going across it, and it's like, that's monitoring the heart rate. So this whole blip right here, if you want to call it that, is actually recording the electrical signals that are going through the heart during heart contractions. So let's actually flip the page and talk about how those electrical signals start and how they propagate.

Electrical Signals of the Heart

The heart beats in response to electrical signals we call action potentials. We'll learn more about these in the chapter on the nervous system. For now, just know that these are electrical signals that are generated by moving ions across the membrane of cells. Now, action potentials in the heart are not transmitted by nerves, like we'll see throughout the nervous system. These action potentials actually move between the cells of the heart through what are called gap junctions. Gap junctions, which you can see right here, are basically direct cell to cell connections, and there are these channels between the cells that ions can flow through; these channels are referred to as connections. You don't need to worry about memorizing any of this; just know that the action potentials in heart cells are moving cell to cell through gap junctions. In fact, there's actually a specialized structure in heart muscle that connects neighboring cells and contains these gap junctions. We call those intercalated discs. And here is an example of some heart tissue, and if you zoom in you can see some intercalated discs between the cells. Now, here we actually have a recording of an action potential, and hopefully, this looks a little familiar to you. Right? That "bloop bloop bloop," that line that you see on the heart rate monitor, right, the thing in the hospital that we just looked at looks a heck of a lot like that. Right? It is in fact measuring what is known as the electric potential, something measured in volts; it's a type of voltage. You don't need to worry about remembering any of this. I just want you to see the similarity between this standard image of an action potential and what we see up here on the heart rate monitor because they're basically measuring the same thing.

Now, how do these action potentials get generated? Well, there is a special part of the heart, a group of cells in the right atrium that are referred to as the sinoatrial node. And basically, these cells are going to be responsible for initiating heart contractions in vertebrates. There's actually a group of cells there that are usually referred to as pacemaker cells, and these are the cells that will control the rate and timing of heartbeats, and they are going to actually start the action potential. They're going to be the initiator of the action potential. Now from the SA node, as it's often referred to, the action potential is going to propagate to what's called the Atrioventricular Node, which is a group of cells that is sort of, almost like the center of the heart. The AV node being pointed out here, so our SA node was up here, the action potential moves down to the AV node.

And at the AV node, something interesting happens. See that signal, that electrical signal is delayed. And the reason for that is we want the atria to have a little extra time to completely empty its blood into the ventricles. So, by having a slight delay in the signal at the AV node, before it moves down into the ventricles, we actually give the atria the time it needs to push all the blood it has out into the ventricles, making the work of the ventricles' contraction more efficient. Now, when this signal goes down into the ventricles, something kind of interesting happens. So, from the AV node, it actually goes down to basically the bottom of the ventricles. And from there, it's propagated up through the ventricles, through these fibers called Purkinje fibers, or Purkinje fibers. You may hear it pronounced, depending on whether you give it a soft or hard 'j'. Anyway, these Purkinje fibers will actually spread the action potential up through the ventricles from the bottom to the top.

Now, the reason for this is the arteries have their openings located kind of at the top of the ventricles. Let me use a different color here, make these arteries like red. So, by having the ventricles start their contraction from the bottom and move it up, you're actually pushing the blood up into these arteries. That's the reason for it. Now, this whole process, this whole electrical process is recorded in what's called an electrocardiogram. Sometimes it's abbreviated EKG, if you're wondering why it's not EKC. EKG comes from the German word, so, you know, kind of confusing there, but it's the same thing, EKG is an electrocardiogram. So this is going to record the electrical activity of the heart, and it's going to be output looking like this. So at the start here we have our SA node, that is going to lead, our SA node is going to initiate the action potential. This is going to be the big contraction of the ventricles, And here we have the relaxation of the muscles. So you don't need to worry about memorizing the different parts of the EKG signal; just wanted to kind of show you how these electrical signals correspond to the phenomena that we've just been talking about. So with that, let's actually flip the page.

Cardiac Cycle

Now let's put together everything we just talked about into one concise package. So, let's begin with diastole, which is, again, the relaxation of the ventricles and atria, which is going to cause them to fill up with blood. Alright. That's what we're seeing here. Right? The ventricles and atria are going to fill with blood. Then, as we can see in our EKG signal, the SA node is going to initiate the action potential, and that's going to cause the atria to contract, or atrial systole, if you will. We have atrial systole, which means that our right atria and left atria are going to empty their blood into the ventricles. We can see that happening there. Now, before our ventricles contract, remember there's a slight delay, and you can actually see that in the signal, it's this little, like, flattening right there. That's the slight delay of the AV node. Right? Delaying the action potential, which is going to allow the atria to completely empty into the ventricles.

Now, in systole, remember, the action potential is going to start at the bottom of the ventricles and move up through the Purkinje fibers, and this causes the ventricles to contract and push blood into the arteries. And in our EKG, that's this big depolarization, a fancy science term for it there. It's the big electrical signal right there, that's that big ventricle contraction. And you can see that happening in these images here. We have our ventricles super full of blood in the left ventricle. These guys are super full of blood in this image right here, and then we're going to get contraction in this image. Right? Those ventricles are getting squeezed and they're going to push the blood into the arteries. Those are, of course, the pulmonary artery and this guy right here, the aorta. So after that, we're going to have relaxation. Right? Go back to diastole, after systole, and that's going to cause the atria and then the ventricles to fill back up with blood, and that relaxation can be seen on the EKG here at the tail end.

Now this whole process is again called the cardiac cycle, and we like to measure it in certain ways. I mean, we look at the electrical signals with EKG, but sometimes we want to know about other facets of the cardiac cycle. One of which is cardiac output, and this is going to be the volume of blood that's pumped per minute by the ventricle. So this is a rate of volume per minute, and it's basically looking at two measurements and putting them together. Those measurements are heart rate, which is heartbeats per minute, or beats per minute. And, you know, this is often written, for example, in music as BPM. Right? If you, you know, like that electronic dance music, you want those high BPMs. You know, obviously, our heart rate we don't want to be too high, but a very interesting thing to note is our heart rate is around 60 beats per minute. Right? A beat every second. And dance music, some of the most popular dance music, is actually at about 120 beats per minute. Double the heart rate. Right? So, kind of an interesting thing to take note of, how, you know, something as obscure as, like, or abstracted from nature as electronic music, adheres to our natural cycles, so to speak.

Now, getting a little distracted, let's get back to cardiac output. The other measure that is involved is stroke volume, which is the volume of blood pumped by a single ventricle contraction. So this is not a rate, this is just a volume. So combining heart rate and stroke volume, you can get cardiac output and see how much blood is being pumped per minute. Let's flip the page.

Blood Pressure

Hello, everyone. In this lesson, we are going to be talking about blood pressure, and how blood pressure changes depending on the different veins, arteries, and capillaries the blood is actually in. Okay. So first off, let's talk about the 2 different types of blood pressure. So I'm sure you've had your blood pressure taken by your doctor, and your doctor gives you these numbers. Generally, they're around 120 over 80. And that actually is your blood pressure, but it's actually 2 different versions of blood pressure. The systolic blood pressure is the top part of the fraction, and the diastolic blood pressure is the bottom part of the fraction. So what's the difference between these 2? Because if you are a pre-med student, you're definitely going to have to know these 2 different types of blood pressure.

The first one, the highest number is gonna be your systolic blood pressure. And this is the highest blood pressure that your arteries should actually experience, and that your heart should actually experience. And systolic blood pressure is going to be taken at the time in the heart phases called systole. And systole is going to be the peak of contractions. So this is the peak of blood pumping out of the ventricles. So whenever the ventricles actively contract and push that blood out of the ventricles, that blood's gonna be of extremely high pressure because those muscles are actively squeezing on it. So Systolic Blood Pressure is the highest blood pressure because this is gonna be the pressure of the blood when the ventricles are actively contracting during the phase called Systole. Systole is going to have a range of healthy blood pressures. During the contraction phase, or systolic phase, the healthy blood pressure should be less than 120 millimeters of Mercury, which is going to be a measurement of pressure. So anything less than 120 millimeters of mercury is good. So around that range is a good systolic blood pressure. Anything above 140 is going to be problematic and is going to cause high blood pressure, which is not good for your body.

Now, the second form of blood pressure is the diastolic blood pressure. This is gonna be the one on the bottom of the fraction when you're given your blood pressure. And this is the lower blood pressure that you experience, and it's the lower blood pressure because it's actually right before the ventricles contract and pump out blood. And this is because this is gonna be the blood pressure of the Diastole Phase in the Heart Contraction Phase. And this is actually the relaxation phase, and this is actually when the chambers of the heart are refilling with blood right before the contraction phase, right before systole. So, this is going to be the very low blood pressure of the heart and of the blood because this is when the heart is relaxing and when the heart is actually refilling with blood, so it can do another cycle of contractions. And this blood pressure also has an optimal blood pressure, and this is anything under 80 millimeters of Mercury. Anything above that is considered high blood pressure and can be dangerous. So that's why they say that you want your blood pressure to be around 120 over 80. So, let me draw this out for you guys. So your fraction would look like this: 120 over 80 millimeters of Mercury. And this one right here is the Systolic. And this one right here is the Diastolic. So that's why you get two numbers whenever you get your blood pressure reading. Okay?

Alright. So now, we use blood pressure to understand how the heart is functioning. So whenever your heart is beating, your arteries actually will bulge because the pressure and the force of the blood is increasing. So you guys can generally put your hands up next to your neck and find an artery, and then it will be pulsing. And that is because with every beat and contraction of your heart, more blood is being pushed through that artery, so it is going to expand. Okay? So the pulse is gonna tell you about the heartbeat, how quickly those contractions are happening, and the blood pressure is going to tell you the pressure of the blood and how much force is being put on that blood. Okay?

So now let's talk about high blood pressure because Americans do have an issue with this, and this is also called hypertension. And hypertension is long-term high blood pressure. This is anything over 120, especially over 140 systolic blood pressure. Anything over that for a very long period of time is called hypertension. Hypertension is generally seen in Americans because we have a very high salt diet, but hypertension can be caused by a high salt diet, a high fat diet, and a lack of exercise. And hypertension can lead to a lot of issues. You can imagine if you have a lot of pressure on your heart and on your arteries and your veins all the time throughout your entire life, you're going to have some issues from that. And this can cause Coronary Artery Disease. This can cause a stroke. This can cause kidney disease. It can cause a whole bunch of issues. So hypertension is generally bad, and this is generally medicated for, or the diet is changed. But I want you guys to know that this is generally caused by diet, but it can also be caused by genetics as well. Some people have a higher propensity to have hypertension than others. Okay?

Alright. So now let's look at this really neat graph, which is gonna be showing us the pressures of these different areas of your cardiovascular system. So what we have is we have the pressures of these different vessels. So you have the aortic pressure, which is gonna be the pressure of your aorta, which is the largest vessel in your body. And the left ventricle is going to push blood into the aorta. And we're going to have the pressure inside of the atria's, and we're gonna have the pressure inside of the ventricles. Now, just so you guys know, whenever you're measuring blood pressure, you're generally measuring the pressure of the aorta, the major arteries of the body. So whenever you're looking at these pressures on this chart, the one we generally go by to measure pressure of the blood is this one in red right here. The Aortic Pressure is generally what we utilize. And if you guys can see, the way we know that is true is because we have the 120 and we have the 80. That's the general healthy blood pressure for an individual, and that's how you know this is the one that we're measuring. We're measuring the aortic Pressure. Okay?

Alright. So, you guys know that we're measuring the Aortic Pressure, and I would like to show you the different phases of the heart's contraction. So, the phase that is happening right here is Systole. This is the contraction phase. All of this right here is the 120. That's gonna be the highest pressure when those ventricles are actively contracting and pushing the blood out. And then once systole is ending, well, the heart is actively refilling with blood. So the pressure here is much lower. And you guys can see that the atria and the ventricles also do change in pressure. You can see the ventricle pressure in black changes substantially. You guys can see that it's way down here, and then during systole, it just jumps up to these huge pressures, and that's because the blood inside of the ventricle is being actively squeezed and being given a ton of pressure, and it's just shooting that blood out of the ventricles, and then it's gonna dramatically drop back down during diastole, and the drop in blood pressure actually aids the heart in pulling more blood into it. So this is basically showing you the different pressures that the different areas of the heart and the different vessels experience. So the aorta is in red, and that is what we generally utilize for blood pressure. That's what we use to read blood pressure is the aortic pressure. But then we also have the ventricular pressure in black, and we have the atrial pressure in blue. And they're all going to vary, but the one that's most dramatic is the ventricular pressure because it greatly jumps up during systole. Okay, guys?

Alright. So now let's go down and let's talk about how the blood vessels are going to deal with this gigantic change in pressure. Because the arteries are the vessels leading away from the heart, and they're going to experience the most intense blood pressure, especially the aorta, which is going to experience the most intense blood pressure because that left ventricle is actively pushing blood into it. So the way they're going to combat this is that arteries have muscle fibers, and elastic fibers help them deal with the high pressure. So this is going to be these muscle fibers and these fibers that help it stretch and help it go back to its normal size during the different contractions of the heart. And the aorta is especially dense with these elastic fibers because it goes through this immense systolic blood pressure, this immense change in pressure every time those ventricles contract. Those fibers are there to help that, to help make sure that these arteries don't burst. They need to be able to withstand this giant change in pressure. So now the arteries are going to have the highest blood pressure. The arteries are the vessels leading away from the heart, and the arteries are the vessels that are actively being pushed blood into. So they have the highest blood pressure. The veins and the capillaries are gonna have the lowest blood pressure. Remember, the capillaries are where n

Cardiovascular Disease

Just like other systems in the body, blood pressure has to be controlled in a homeostatic manner. Baroreceptors are pressure sensors that help detect blood pressure in the heart and the arteries. Obviously, a certain level of blood pressure needs to be maintained, and it is affected by factors such as blood vessel dilation and constriction, as well as blood volume. For instance, having a higher blood volume can generate greater pressure. Conversely, constricting blood vessels can create greater pressure even with a lower blood volume. There are many different mechanisms to essentially tweak and fine-tune to achieve the optimal blood volume and pressure. For example, the heart can increase cardiac output in response to low blood pressure. If the blood pressure is low, it will pump more blood to counteract that effect. Additionally, if the body is experiencing low blood volume, or for other reasons as well, blood can be diverted to important tissues. During a fight or flight response, blood is diverted away from digestive tissues because in such scenarios, digesting a meal is less crucial. This is accomplished by constricting specific arterioles to ensure that more blood goes to more critical tissues, like muscles during a fight or flight scenario. This tweaking can also be beneficial when experiencing a lower blood volume, ensuring that critical areas such as the brain still receive sufficient blood supply. By constricting blood vessels, the body can regulate blood delivery to various tissues.

Moreover, veins can constrict to divert more blood volume towards the heart and arteries. These effects do not have to apply across all blood vessels. For example, veins can constrict to provide less space for blood, ensuring more blood is present in the heart and arteries, which is critical as arteries deliver blood to the tissues, whereas veins bring it back to the heart. Furthermore, if the blood pressure becomes too high, dilating blood vessels can initiate a drop in blood pressure by creating less resistance to the flow of blood.

Additionally, there are several types of regulators connected to the heart, represented by various neuronal or nervous system connections that can modify the pumping of blood and influence blood pressure. Aside from these connections, there are numerous other mechanisms that provide a homeostatic system to fine-tune blood pressure and blood volume.

When things go wrong, we refer to diseases that affect the heart or vasculature as cardiovascular disease. One of the most common conditions is arteriosclerosis, the hardening of the arteries due to an accumulation of fat deposits. Cholesterol plays a significant role in this; it is a crucial molecule used in producing steroid hormones and in maintaining membrane fluidity in cells. Cholesterol is commonly described in layman terms as "good" and "bad." Low-density lipoprotein (LDL) is often referred to as 'bad cholesterol' because it delivers cholesterol in the body, leading to those harmful deposits. High-density lipoprotein (HDL), on the other hand, is termed 'good cholesterol' because it scavenges excess cholesterol, acting like a cleanup crew and helping prevent arteriosclerosis. A myocardial infarction, known commonly as a heart attack, occurs when one of the coronary arteries is blocked, leading to the damage of heart muscle tissue. Lastly, a stroke, which is damage to nervous tissue in the brain, is often caused by a lack of oxygen being delivered to the brain due to a blocked or ruptured artery in the brain. All these issues are severe and underline the necessity of maintaining heart health to prevent such outcomes.

That's all for this video. I hope it doesn't give you nightmares. Keep your hearts healthy, guys.

Here’s what students ask on this topic:

What is the cardiac cycle and its phases?

The cardiac cycle is the sequence of events in the heart from the beginning of one heartbeat to the beginning of the next. It consists of two main phases: systole and diastole. Systole is the contraction phase where the heart muscles contract to pump blood out of the ventricles into the arteries. Diastole is the relaxation phase where the heart muscles relax, allowing the chambers to fill with blood. During diastole, blood flows from the veins into the atria and then into the ventricles. The cycle ensures continuous blood flow throughout the body, maintaining oxygen and nutrient delivery to tissues.

How do electrical signals control heart contractions?

Heart contractions are controlled by electrical signals known as action potentials. These signals originate in the sinoatrial (SA) node, located in the right atrium. The SA node generates action potentials that propagate through the heart muscle via gap junctions and intercalated discs. The action potential travels to the atrioventricular (AV) node, where it is briefly delayed to allow the atria to empty blood into the ventricles. It then moves down to the ventricles through Purkinje fibers, causing the ventricles to contract from the bottom up, efficiently pumping blood into the arteries. This process is recorded in an electrocardiogram (EKG).

What is the difference between systolic and diastolic blood pressure?

Systolic blood pressure is the higher number in a blood pressure reading and represents the pressure in the arteries when the heart's ventricles contract (systole). Diastolic blood pressure is the lower number and indicates the pressure in the arteries when the heart is in the relaxation phase (diastole), refilling with blood. Optimal blood pressure is around 120/80 mmHg, with 120 mmHg being the systolic pressure and 80 mmHg being the diastolic pressure. Maintaining these values is crucial for cardiovascular health, as high blood pressure (hypertension) can lead to serious health issues.

What are the consequences of hypertension?

Hypertension, or high blood pressure, can lead to several serious health conditions. Prolonged high blood pressure can cause damage to the arteries, leading to atherosclerosis (hardening of the arteries) due to fat deposits. This increases the risk of heart attacks (myocardial infarctions) and strokes, as blocked or ruptured arteries can impede blood flow to the heart or brain. Hypertension can also cause kidney disease and other cardiovascular issues. It is often managed through lifestyle changes such as diet and exercise, and sometimes medication is required to maintain healthy blood pressure levels.

How is cardiac output calculated?

Cardiac output is the volume of blood pumped by the heart per minute. It is calculated using the formula:

$\mathrm{Cardiac} \mathrm{Output}=\mathrm{Heart} \mathrm{Rate} \times \mathrm{Stroke} \mathrm{Volume}$

Heart rate is the number of heartbeats per minute (bpm), and stroke volume is the amount of blood pumped by the ventricles in one contraction. For example, if the heart rate is 70 bpm and the stroke volume is 70 mL, the cardiac output would be 4900 mL/min (or 4.9 L/min). This measurement helps assess the efficiency of the heart's pumping ability.